1. Field of the Invention
The present invention relates to a memory controller, e.g., a memory controller for controlling the operation of a nonvolatile semiconductor memory chip.
2. Description of the Related Art
With the recent rapid spread of digital cameras and portable audio players, demands for large-capacity nonvolatile semiconductor memories are increasing, and NAND flash memories (to be also simply referred to as flash memories hereinafter) are widely used as the nonvolatile semiconductor memories.
In the NAND flash memory, data is erased from a plurality of memory cells at once. This erase unit will be called a memory block hereinafter. The memory block includes a plurality of NAND cells. Each NAND cell has a selection transistor ST1 having a drain connected to a bit line, a selection transistor ST2 having a source connected to a source line, and a plurality of memory cell transistors MT having current paths connected in series between the source of the selection transistor ST1 and the drain of the selection transistor ST2.
In the conventional NAND flash memory described above, data is written by selecting a certain word line. This technique is described in, e.g., “Jpn. Pat. Appln. KOKAI Publication No. 2005-285184” or “SmartMedia™ (registered trademark) Physical Format Specification Ver 1.21, issued by SSFDC Forum Technical Meeting, May 19, 1999”. However, this technique has the problem that the reliability of the system deteriorates due to the loss of important data.